180_Semantic_Analysis

180_Semantic_Analysis - CS143 Handout 18 Summer 2011 July...

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CS143 Handout 18 Summer 2011 July 11 th , 2011 Semantic Analysis What Is Semantic Analysis Parsing only verifies that the program consists of tokens arranged in a syntactically valid combination. Now we’ll move forward to semantic analysis , where we delve even deeper to check whether they form a sensible set of instructions in the programming language. Whereas any old noun phrase followed by some verb phrase makes a syntactically correct English sentence, a semantically correct one has subject-verb agreement, proper use of gender, and the components go together to express an idea that makes sense. For a program to be semantically valid, all variables, functions, classes, etc. must be properly defined, expressions and variables must be used in ways that respect the type system, access control must be respected, and so forth. Semantic analysis is the front end’s penultimate phase and the compiler’s last chance to weed out incorrect programs. We need to ensure the program is sound enough to carry on to code generation. A large part of semantic analysis consists of tracking variable/function/type declarations and type checking. In many languages, identifiers have to be declared before they’re used. As the compiler encounters a new declaration, it records the type information assigned to that identifier. Then, as it continues examining the rest of the program, it verifies that the type of an identifier is respected in terms of the operations being performed. For example, the type of the right side expression of an assignment statement should match the type of the left side, and the left side needs to be a properly declared and assignable identifier. The parameters of a function should match the arguments of a function call in both number and type. The language may require that identifiers be unique, thereby forbidding two global declarations from sharing the same name. Arithmetic operands will need to be of numeric—perhaps even the exact same type (no automatic int -to- double conversion, for instance). These are examples of the things checked in the semantic analysis phase. Some semantic analysis might be done right in the middle of parsing. As a particular construct is recognized, say an addition expression, the parser action could check the two operands and verify they are of numeric type and compatible for this operation. In fact, in a one-pass compiler, the code is generated right then and there as well. In a compiler that runs in more than one pass (such as the one we are building for Decaf), the first pass digests the syntax and builds a parse tree representation of the program. A second pass traverses the tree to verify that the program respects all semantic rules as well. The single-pass strategy is typically more efficient, but multiple passes allow for better modularity and flexibility (i.e., can often order things arbitrarily in the source program).
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2 Types and Declarations We begin with some basic definitions to set the stage for performing semantic analysis. A
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180_Semantic_Analysis - CS143 Handout 18 Summer 2011 July...

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